The present invention relates to communications transmitters, such as cellular telephone transmitters, and more particularly to communications transmitters and wireless communications mobile terminals using frequency synthesizers to generate a phase-encoded signal.
In wireless communications, it is common for signals to be transmitted with both amplitude and phase encoding. For instance, one standard for cellular communications common in North America, TIA/EIA-136, calls for amplitude and phase modulation for encoding information on a carrier frequency. This standard is well known to those of ordinary skill in the art. For TIA/EIA-136, and other standards, it is common to employ an I/Q modulator architecture that uses one or more mixing stages to take a transmit signal from baseband to the appropriate carrier frequency. In these architectures, the Inphase (I) and Quadrature (Q) signal components representing the data to be transmitted are generated at baseband. From baseband, the I and Q signals are mixed to typically two or more intermediate frequencies (in series) and then to the carrier frequency.
In one aspect of the present invention, a variable frequency synthesizer is used during generation of a phase-encoded signal. As with traditional frequency synthesizers, the output of the frequency synthesizer is based at least in part on an input reference signal. In the present invention, the reference signal input to the frequency synthesizer is not held stable and constant, but is instead deliberately varied to control the output of the frequency synthesizer. This varying reference signal, referred to herein as the phase reference signal, is derived from traditional inphase and quadrature signals, as modified by a phase distorter. The phase distorter applies a pre-distortion to the inphase and quadrature signals so as to compensate for band-limiting effects of the frequency synthesizer. The signal transmitted is then based on the phase-encoded signal.